Conventionally, congestion has been occurred during transmission in a packet network to cause loss of packet data due to collision when the network is disorganized. When the congestion is generated, the network is disorganized more and more, and there is increased packet delay in the case of retransmission of all the lost packets.
Therefore, it is considered that a large number of memories are arranged at a receiving side, and playback is started after receiving all the information on moving images when the packet transmission is used for communication of the moving images. However, the above method may not be applied for real-time communication as there is large delay.
In order to solve the problem, a transmission method disclosed in the prior Japanese Patent Application Publication No. Hei-10 (1998)-262245 has been proposed. This transmission method adopts hierarchical coding for coding of moving pictures, forms packets according to their significance, and only information which may be transmitted in one moving-image frame cycle is sent in the decreasing order of significance.
In the transmission method using the hierarchical coding, there is a less disorganized network, as only information with higher significance is transmitted, and one with lower significance is discarded when the network is disorganized. Even when packets are discarded, playback may be performed with less degradation in picture quality at a receiving side, using packets with higher significance.
However, there has been a problem in a conventional apparatus that packets, which have been received late for playback time (display time) are discarded, and discarded packets may not be preserved in a receiving apparatus as buffer control is performed in order to display images.
Moreover, there has been another problem that transmission bands are uselessly used, as there is performed retransmission of data which are discarded, and, accordingly, not played back when the retransmitted packets are not in time for the playback time, even if the retransmission of packets is requested by a receiving side.